Air filter coating compositions containing mixture of silica and magnesium silicate gelling agents

ABSTRACT

This application discloses triorganic phosphate coating compositions for viscous-impingement air filters in which the triorganic phosphate is thickened by a combination of about 0.1 to 1.0 parts of a magnesium silicate, with at least 10 percent of its particles which are less than 1 micron in any dimension and whose silica to magnesia ratio is about 1.05 to about 1.30 to 1, and 0.1 to 3 parts of pyrogenic silica having a particle size no greater than 0.025 microns. These gel compositions have good high-temperature viscosity stability and exhibit low syneresis losses. Satisfactory gels can be prepared using as little as one percent pyrogenic silica with 0.5 percent of the magnesium silicate.

United States Patent Lynch, Jr. et al.

[ 1 Feb. 1,1972

[54] AIR FILTER COATING COMPOSITIONS CONTAINING MIXTURE OF SILICA ANDMAGNESIUM SILICATE GELLING AGENTS [72] Inventors: Charles Andrew Lynch,Jr., Sevema; James P. Hamilton, Pasadena, both of Md.

73 Assignee: FMC Corporation, New York, N.Y.

22 Filed: Sept. 28, 1967 211 Appl.No.: 671,222

[52] US. Cl ..252/88, 23/110, 23/182,

252/184, 252/317 {51] Int. Cl ..C0lb 33/14, C01b 33/22, C09k 3/22 [58]Field ofSearch ..252/88,184,315, 316,317,

Primary Examiner-John T. Goolkasian Assistant Examiner-M. E. McCamishAttorney-Milton Zucker, Eugene G. Seems and Charles C. Fellows [57]ABSTRACT This application discloses triorganic phosphate coatingcompositions for viscous-impingement air filters in which the trierganicphosphate is thickened by a combination of about 0.1 to 1.0 parts of amagnesium silicate, with at least 10 percent of its particles which areless than 1 micron in any dimension and whose silica to magnesia ratiois about 1.05 to about 1.30 to l, and 0.1 to 3 parts of pyrogenic silicahaving a particle size no greater than 0.025 microns. These gelcompositions have good high-temperature viscosity stability and exhibitlow syneresis losses. Satisfactory gels can be prepared using as littleas one percent pyrogenic silica with 0.5 percent of the magnesiumsilicate.

3 Claims, No Drawings AIR FILTER COATING COMPOSITIONS CONTAINING MIXTUREOF SILICA AND MAGNESIUM SILICATE GELLING AGENTS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to novel,bodied, thixotropic triorganic phosphate gel compositions suitable forcoating dust filter media in air filters.

2. Description of the Prior Art Viscous-impingement-type industrial airfilters contain porous filter media, or mats, through which air ispassed. The filter medium is coated with a stable, nonvolatile liquidwhich entraps dust particles as air is passed through the filter andprevents the dust particles from becoming reentrained in the air stream.However, loss of this liquid coating during use results in a loweringofthe air cleaning capacity of the filter. The liquid may be blown off thefilter media by high-air velocities, encountered, for example, withdiesel engine filters. Moreover, when a filter medium is tightly woundaround spools, as in the recently developed automatic replacementfilters, the liquid tends to be squeezed out ofthe filter media.

In the past, these losses have been reduced by bodying the liquid with asemisoluble resin or other gelling agent. However, these resin-liquidcompositions tend to decrease in viscosity at elevated temperatures,especially at the temperatures reached with diesel engine filters. 1fgelling agents, such as finely divided silica are used, as much as 7percent, based on the mixture, is ordinarily required for satisfactorilythickening a suitable liquid such as a triorganic phosphate. This amountof silica in the liquid can clog spray nozzles and associated filtersgenerally used in coating the mats. Furthermore, the mixture becomesthicker upon aging due to syneresis resulting in the reduction of theability to entrap dust particles. Syneresis may also occur duringstorage causing the liquid to separate from the gel structure. Theseparated triorganic phosphate will then drain out of the mat leaving ahigher concentration of gel solids on the surface of the mat. Theoverall effect is a loss in dust filtering capacity.

Excellent triorgano-phosphate gels at lower silica levels for use onviscous-impingement-type air filters can be prepared by usingsurfactants with low levels of silica, as described in U.S. Pat. No.3,297,460.

SUMMARY OF THE INVENTION We have now discovered improved coatingcompositions for viscous-impingement air filters which comprise: (a) 100parts by weight ofa triorganic phosphate of the formula .11..-. has...

in which n is -l the alkyl radical is ofone to carbon atoms and the arylradical is derived from tar acids; (b) 0.1-3 parts by weight ofsubstantially pure pyrogenic silica having a particle size not greaterthan 0.025 microns and (c) 0.1 to 1.0 parts by weight of amagnesiumsilicate which has at least 10 percent of the particles lessthan 1 micron in any dimension and a silica to magnesia (SiO :MgO) ratioof 1.05 to 1.30 to l. The pyrogenic silica alone does not thicken thetriorganic phosphate sufficiently to form satisfactory coatingcompositions. Triorganic phosphate compositions containing pyrogenicsilica and component (c) are thickened enough to be useful as coatingsfor air filter media that do not drain from the filter at roomtemperature and maintain a sufficiently high viscosity at elevatedtemperatures to perform satisfactorily. We have evaluated a number ofcombinations of the pyrogenic silica and magnesium silicate and haveproduced satisfactory gels with as little as 1 percent pyrogenic silicaand 0.5 percent by weight of magnesium silicate. Quite surprisingly, airfilters coated with gel compositions made using the combination ofgellants, pyrogenic silica and magnesium silicate, have unexpectedly lowsyneresis losses and very good high-temperature viscosity stability;this is very surprising since the pyrogenic silica alone is a veryinefficient gellant.

and compositions gelled with a magnesium silicate alone havehigh-syneresis losses.

DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS Thetriorganic phosphates which were useful in accordance with thisinvention are those having the formula y -0)" E-O-wryl) in which n is0-1, the alkyl radical is l to 10 carbon atoms and the aryl radical isderived from tar acid. The term tar acid" is intended to include thoseacids which can be extracted with dilute caustic soda from tars, suchas" coal tar, common wood tar, petroleum tar and lignite tar. Crude taracid extracts usually contain phenol, ortho-cresol, meta-cresol,para-cresol and isomeric xylenols, and they can be fractionated to givevarious grades of these products. Triorganic phosphates of particularuse practicing this invention are termed tricresyl phosphates. The termtricresyl phosphate is intended to include esters made from phosphorusoxychloride and mixtures of alkylphenols known as cresylic acids" whichmay be derived from coal tar, wood tar or petroleum distillate. Suitablealkyl radicals include methyl, ethyl, propyl, iso-propyl, butyl, amyl,isohexyl and ethylhexyl, as well as others. Preferably the phosphate istricresyl phosphate.

The silica that we used in preparing the high-viscosity compositions ofthis invention is of that type known as pyrogenic silica. This type ofsilica is prepared by reacting almost pure silicon tetrachloride andwater in a vapor phase to produce silicon dioxide and hydrogen chloride.The silica so produced is a very pure powder, of the order of 99 percentpure having a particle size not greater than about 0.025 microns.Pyrogenic silica having particle sizes as small as about 0.007 micronsis presently available and useful in practicing this invention.Preferably, the pyrogenic silica has a particle size of about 0.015 to0.020 microns.

Magnesium silicates useful in preparing the compositions of thisinvention are described in U.S. application Ser. No. 436,304, filed Mar.1, 1965 abandoned in favor favor of continuation-in-part applicationSer. No. 701,838 which is now U.S. Pat. No. 3,458,393. The magnesiumsilicates described in this patent application have at least 10 percentof the particles less than 1 micron in any dimension and have an SiO:MgO ratio in the range of between 1.05 to 1 and 1.30 to 1 In all casesit is necessary that the magnesium silicate contain about 10 percent byweight of particles of submicron particle size in all dimensions. Wehave found 0.1 to 1 parts magnesium silicate to give usefulcompositions. Higher levels of magnesium silicate can be used, howeverthe gels are too viscous for application by conventional techniques.

The coating compositions of this invention were prepared by dispersingthe gelling agents in the triorganic phosphate in a high-speedlaboratory blender for a short period of time, sometimes as short as 1.5minutes. As far as is known, any method of dispersing solids in liquidsto produce coating compositions of this type may be used in practicingthis invention.

A small amount of a surface active agent incorporated in a coatingcomposition of this invention will increase the viscosity of thecomposition. This permits the use of less gellant to obtain a desiredviscosity. As little as 0.1 percent by weight, based on the total weightof the mixture, results in some viscosity improvement; however, 0.3 to2.0 surface active agent is preferred. Use of 2.5 percent or more of asurface active agent results in useful coating compositions, however thehigh viscosities obtained are not generally required for commercial use,and thus in most cases, the additional cost is not warranted.

As the surface active agent, nonionic surfactants, such aspolyoxyethylene sorbitan monooleate, polyoxyethylene sor bitantristearate and sorbitan monostearate, and cationic surfactants, such asalkyl-trimethylammonium chloride, dialkyl EXAMPLE 1 A series of gelswere prepared by stirring different ratios of the two gellants,pyrogenic silica and magnesium silicate, into 60 r.p.m Vis./temp. at 6r.p.m.:

I No gel. *4-Repeat of Example 4 in Table I.

40 .IIIIIIIIIIIIIIIII 11.

dimethylammonium chloride, octadecenyl trimethylammonia trlorganlcphosPhme' tricr?$y| phosphategfor perild um chloride, andtetraethylammonium bromide, are extremeof j hlgh'speed,mlxer' Comparanveexamples were ly effective in increasing viscosity of the organicphosphate also run m which only a Smgle gellam was used reportedcomposition containing the pyrogenic silica and magnesium as "1 examplesC and The m f the silicate gelling agents. Glycols, such as ethyleneglycol and f fm and test l obfamed are li m [able propylene glycol andpolyglycols Such as polyethylene glycols V scoslties were determinedusing 21 Brook leld Model LV F and polypropylene glycols are alsosuitable surface active v'scometer 3 l l at 6 1 5 a agents for thisinvention. A minor quantity of water in the losses weregetermmed y z g Yl i 0 m gelled organic phosphate compositions of this invention also anu ncoate g, cup an medsurmg l e welg t 9 a increases the viscosity ofthe gel. If an anionic surfactant is varying peno so used, a smallamount of water must be present. Glycerol is also EXAMPLEZ an effectivesurface active agent. If it IS desired, mixtures of different surfaceactive agents may be used. In all of these sur- A second series ofsamples were prepared to show the effect factant treated gels, theviscosity increase is stable for many of a surfactant on gelcompositions gelled with mixed gellant days. combinations. Theformulations were prepared by mixing the The following examples,illustrating the novel filter medium gellants, surfactants and tricresylphosphate in a high-speed coating compositions of this invention, arepresented with no laboratory mixer for a short period of time. Thecomposition intent that the invention be limited thereto. All parts andperdetails and the viscosity characteristics of the various mixturescentages are by weight. For comparison, the ungelled tricresyl f ricreyl p p py g Silica. magnesium Silicate phosphate used in the exampleshad a viscosity of approxiand surfactant are found in table ll. Thecomparison example mately I20 cps. at room temperature. 1 usingpyrogenic silica at a single level is included in the tables.

TABLE 1 Comparison examples Examples A B o D 1 2 a 4 5 5 7 IngredientsTricresyl phosphate 09. 99. 5 JO. 0 97 100 100 100 100 100 98. 9Magnesiumslllcate 0.25 0.5 1.0 0 25 0.25 0.25 0 5 0 5 0.5 1.0 Pyrogemesllica 3 1 0 2.0 3.0 1 0 2 0 3.0 0 1 Viscosity propertles (cps):

Vis. (initial):

6 r.p.n1 2, 500 6, 500 10, 000 276 3, 500 6, 000 6, 900 6, 900 7, 200 9,000 31, 000 00 r. .m 650 1,650 2,250 220 850 1,260 1 360 1, 940 2,5002,760 4,15 Vis. (24 hrs.):

6 r.p.m 2, 100 10,000 14, 500 6,500 13, 500 15,500 60 r.p.m 500 1, 9501, 800 2, 200 2, 400 Vis. (7 days):

6r.p.m 1,800 6,500 12,000 450 3.1 2.7 2.9 3.3 23 2.3 10.0 7.9 7.7 9.1 616.4 22. 5 16. 7 14. 6 18. 1 12. 6 11. 3 1 At 23 C.

TABLE II Comparison examples Examples E F 4 8 0 10 11 1 Ingredients:

Tricresyl phosphate 100 100 00 100 100 100 100 100 Pyrogenio sllioa 2. 52. 5 1 0 1. 0 1.0 1.0 1.0 2. 5 Magnesium silicate, 0 5 0.5 0.5 0.5 0.50.5 Polyoxyethylene sorbltan monooleate 2. 5 0.33 0.67 1. 0 1. 7 2. 5Vlscosity propertles (cps) Vis. (initlal):

6 r.p.m 32, 000 6, 900 15,500 17, 600 10, 500 22,000 65, 000 60 1'.p.m4,850 1, 040 a, 050 a, 400 a, 500 4,150 0, 350 Vis. (24 hrs.):

6 r.p.m 65,000 1 10, 000

The viscosities in the various coating compositions were determinedusing a Brookfield Viscometer (Model LVF. using the No.4 spindle at 6and 60 rpm). The viscosity of the mixtures was also measured at variouselevated temperatures using the Brookfield viscometer Model LVF with aNo. 4 spindle at 6 rpm.

EXAMPLE 3 Triorganic phosphate coating compositions were prepared fromseveral commercially available triorganic phosphates which were mixedwith pyrogenic silica and magnesium silicate. The compositions wereprepared as described in example l. The compositions, ingredients, andBrookfield Viscosities (Model LVF, No. 4 spindle at 6 rpm.) at roomtemperature and various elevated temperatures are presented in tablePyrogenic silica 3 3 3 3 Magnesium silicate Viscosity tcps.) 6 r.p.m. i1.500 900 1,500 2.200

Initial 60 rpm. 2.560 300 480 600 Viscosity (cps. at 6 r.p.n1.l 22 T C.15.500 900 1.500 3.000

EXAMPLE 4 Triorganic phosphate coating compositions were prepared fromtricresyl phosphate mixed with pyrogenic silica and magnesium silicatetogether with small amounts ofa surface active agent. The surface activeagents evaluated include nonionic surfactants, cationic surfactants,glycols, polyglycols and water. The coating compositions were preparedas described in example 1. The compositions, ingredients, and BrookfieldViscosities (Model LVF using a No. 4 spindle at 6 and 60 r.p.m.) at roomtemperature are presented in table IV.

TABLE IV 'lricresyl phosphate 100 100 100 100 100 100 Pyrogenicsilica 1. 0 1. 0 1. 0 1.0 1. 0 1.0 Magnesium silicate. 0. 0. 5 0. 5 0. 60.6 0. 5 Water 1. 0 Ethylene glycol 1.0 Polyethylene glycol (400molecular Wt.) 1. 0 Glycerol 1. 0 Tetraethylarnmonium bromide 1. 0Viscosity (cps), room to mp;

6 r.p.rn 6,900 48, 000 30, 000 22, 000 40, 000 32,000 60 r.p.rn. 8, 0006, 800 2, 900 5, 300 500 Example 4 from Table I. 7 V

EXAMPLE 5 A series of coating compositions containing cresyl diphenylphosphate gelled with pyrogenic silica and magnesium silicate, with andwithout a surfactant, were prepared according to the procedure ofexample 2. The composition details and viscosity characteristics of thevarious compositions are included in table V. The viscosities of thevarious compositions were determined at room temperature, using aBrookfield Viscometer, (Model LVF using a No.4 spindle at 6 and 60 rpm).

TABLE V Cresyl diphenyl phosphate 97.5 97.25 97.0 96 S Pyrogenic silica2.0 2.0 2.0 2.0 Magnesium silicate 0.5 0.5 0.5 0.5 Polyoxyethylenesorbitan monoolcate 0.25 0.5 1.0 Viscosity. room temp.

cps. 6 rpm. 6500 46.000 51.000 54.000 60 rpm. 1.650 3.800 4.000 5.150

EXAMPLE 6 Coating compositions containing methyl diphenyl phosphate andisodecyl diphenyl phosphate gelled with pyrogenic silica and magnesiumsilicate, were prepared according to the procedure of example 2. Thecomposition details and viscosity characteristics of the compositionsare included in table VI. The viscosities were measured with aBrookfield viscometer. (Model LVF using a No. 4 spindle at 6 and 60r.p.m.).

TABLE Vl Methyl diphenyl phosphate I00 Isodecyl diphenyl phosphate I00Pyrogenic silica 3 3 Magnesium silicate 0.5 0.5

Viscosity (cps) 6 r.p.rn. 500 500 initial 60 r.p.m. 230 250 Viscosity(cps. at 6 rpm.) 25 T C 500 500 As will be apparent to those skilled inthe art, numerous modifications and variations of the embodimentsillustrated above may be made without departing from the spirit of theinvention or the scope ofthe following claims.

What is claimed is:

l. A triorganic phosphate composition for viscous-impingement airfilters which comprises: (a) 100 parts by weight of a triorganicphosphate of the formula in which n is 0-1, the alkyl radical is one to10 carbon atoms and the aryl radical is derived from tar acids; (b) 0.]to 3 parts by weight of substantially pure pyrogenic silica having aparticle size not greater than 0.025 microns; and (c) 0.1 to 1.0 partsby weight of magnesium silicate which has at least 10 percent by weightof its particles present as submicron particle sizes in all dimensionsand which have a silica to magnesia (SiO :MgO) ratio between L05 to land 1.30 to l.

2. The coating composition of claim 1 in which the triorganic phosphateis tricresyl phosphate and the pyrogenic silica has a particle sizeof0.0l5 to 0.020 microns.

3. The coating composition of claim 1 further comprising 0.1 to 2.5percent by weight, based on the total mixture of a material selectedfrom the group consisting of nonionic surfactants, cationic surfactants,glycols, glycerol, polyglycols and water.

2. The coating composition of claim 1 in which the triorganic phosphateis tricresyl phosphate and the pyrogenic silica has a particle size of0.015 to 0.020 microns.
 3. The coating composition of claim 1 furthercomprising 0.1 to 2.5 percent by weight, based on the total mixture of amaterial selected from the group consisting of nonionic surfactants,cationic surfactants, glycols, glycerol, polyglycols and water.